With an increase in capacity of information processing in recent years, various information recording technologies have been developed. In particular, the surface recording density of an HDD using magnetic recording technology is continuously increasing at an annual rate of approximately 100%. In recent years, an information recording capacity exceeding 160 GB per one perpendicular magnetic recording medium with a 2.5-inch diameter for use in an HDD or the like has been desired. To fulfill such demands, an information recording density exceeding 250 Gbits per one square inch is desired to be achieved.
To attain a high recording density in a perpendicular magnetic recording medium for use in an HDD or the like, a perpendicular magnetic recording medium of a perpendicular magnetic recording type has been suggested in recent years. In the perpendicular magnetic recording type, the axis of easy magnetization of a magnetic recording layer is adjusted so as to be oriented in a direction perpendicular to the base surface. Thus, in the perpendicular magnetic recording type, compared with the in-plane recording type, a thermal fluctuation phenomenon can be suppressed, and therefore the perpendicular magnetic recording type is suitable for increasing the recording density.
And, in the perpendicular magnetic recording type, the magnetic recording layer can have a granular structure in which a non-magnetic substance (mainly an oxide) is subjected to segregation between magnetic particles (magnetic grains) to form a grain boundary part. With this, magnetic particles can be isolated and made finer, thereby improving an SNR (Signal/Noise Ratio) and a coercive force Hc. Patent Document 1 describes a configuration in which magnetic particles are epitaxially grown to form a granular structure in a columnar shape.
Also, there is a case of providing a non-magnetic granular layer (which may also referred to as a layer for promoting finer grains or an onset layer) of a non-magnetic granular structure in which SiO2 is subjected to segregation in the grain boundary of a non-magnetic metal of CoCr (Patent Document 2). The non-magnetic granular layer is formed above a Ru ground layer. However, the Ru ground layer is of crystals that are not isolated but are continuing. Therefore, when a granular layer is tried to be formed on the Ru layer, separation is not necessarily sufficient at an initial stage. For this reason, crystal orientation is broken on the base of the granular columns, and the state becomes such that the base is horizontally spread to be coupled together. Such breakage in crystal orientation invites a decrease in SNR and coercive force Hc. Also, coupling of the crystal particles invites a decrease in SNR after all. Thus, the non-magnetic granular layer is to promote separation and isolation of granular (magnetic particles) of the magnetic recording layer even when they are connected together, by forming a granular bottom portion with a non-magnetic substance.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-217107    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2006-268972